1. Field of the Invention
The present invention relates to a memory cell, and more particularly relates to a static random access memory cell and a semiconductor integrated circuit device using the same.
2. Description of the Related Art
In recent years, a semiconductor integrated circuit device includes a semiconductor memory to write and read a data, and this is used in a computer system, a portable telephone and the like. As the semiconductor memory, DRAM (Dynamic Random Access Memory), SRAM (Static Random Access Memory), a non-volatile memory and the like are known.
A gate array is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 6-236688). In this first conventional example, a gate array device is composed of basic cell transistors having a same dimension, and the operation speed and stability of a high-speed memory cell is accomplished at small electric power consumption.
A semiconductor device is disclosed in Japanese Laid Open Patent Application (JP-P2001-93993A). In this second conventional example, a static memory operates at as low voltage as a power source voltage of about 1 V, and a small electric power consumption and improvement of an operation speed is accomplished, while avoiding a leakage current at a waiting state through a sub threshold current.
A semiconductor memory device of a static type is disclosed in Japanese Laid Open Patent Application (JP-P2002-32990A). In this third conventional example, even if a defective memory cell is produced, the memory device can be relieved while a standby current is suppressed.
Japanese Laid Open Patent Application (JP-P 2002-42476A) discloses a semiconductor memory device of a static type where a write margin can be set.
Japanese Laid Open Patent Application (JP-A-Heisei, 5-144265) discloses a semiconductor memory device as a fourth conventional example which data can be cleared from or set in a memory cell at a high speed while a higher integration is attained.
Japanese Laid Open Patent Application (JP-P2001-525098A) discloses a method and an apparatus as a fifth conventional example which enlarge the width of a write margin and a read stability margin of a memory cell, without requiring any voltage higher than a power source voltage and any voltage lower than a ground voltage.
FIG. 1 shows the configuration of a semiconductor integrated circuit device in which a typical SRAM cell array 110 is provided. The memory cell array 110 is provided with memory cells 101 of m columns and n rows (m and n are integers of 2 or more). Word lines WL1 to WLn are connected to the n rows of the memory cells 101 in the memory cell array 110, respectively, and n pairs of digit lines DT1 and DB1 to DTm and DBm are connected to m columns of the memory cells 101 in the memory cell array 110, respectively.
The memory cell 101 on the ith column and jth row (i=1, 2 to m, j=1, 2 to n) of the memory cell array 110 includes inverters 111 and 121 and access transistors (path gate transistors) N11 AND N21, which are N-channel type MOS (NMOS) transistors, as shown in FIG. 2.
The inverters 111 and 121 are connected to each other in a crosswise manner to form a flip-flop. The inverter 111 includes a load transistor P11 that is a P-channel type MOS (PMOS) transistor, and a drive transistor N12 that is an NMOS transistor. The inverter 121 includes a load transistor P21 that is a PMOS transistor, and a drive transistor N22 that is the NMOS transistor. In the access transistor N11, its gate is connected to the word line WLj, its drain is connected to the digit line DTi, and its source is connected through a node QT to a drain of the drive transistor N12, a gate of the drive transistor N22, a drain of the load transistor P11, and a gate of the load transistor P21. In the access transistor N21, its gate is connected to the word line WLj, its drain is connected to the digit line DBi, and its source is connected through a node QB to a gate of the drive transistor N12, a drain of the drive transistor N22, a drain of the load transistor P21, and a gate of the load transistor P11. The sources of the load transistors P11 and P21 are connected to a power source, and a power source voltage VDD is supplied from the power source to the sources. The sources of the drive transistors N12 and N22 are grounded, and a ground voltage GND is supplied to the sources.
In the memory cell 101 composed of the 6 transistors shown in FIG. 2, a ratio of the current capability between the access transistor N11 or N21 activated through the word line WLj and the drive transistor N12 or N22 in a latching section is usually assumed to be about 1:3. Also, the current capability of the load transistors P11 and P21 in the latching section are assumed to be equal to or less than those of the access transistors N11 and N21 to achieve a fast stable operation.
However, when respective transistors have a same current capability as in a gate array and the like, there is a case that an operation margin becomes extremely small.
For the above reason, in the above first conventional example (Japanese Laid Open Patent Application (JP-A-Heisei, 6-236688)), as shown in FIG. 3, a plurality of resistance element PMOS transistors P100 are provided between the inverters 111 and 121 of the memory cell 101 and the power source VDD. The plurality of resistance element transistors P100 are connected in series. The gates of the resistance element transistors P100 are grounded, and the resistance element transistors P100 are always on-states. Thus, a resistance unit R100 is formed. The power source voltage VDD is supplied to a source of a first one of the series-connected resistance element transistors P100. A drain of the last one of the series-connected resistance element transistors P100 is connected through a wiring L100 to a node Q1. The source of the load transistor P11 of the inverter 111 and the source of the load transistor P21 of the inverter 121 are connected to the node Q1.
In this way, according to the memory cell 101 of the first conventional example, it is possible to reduce the current capability of the load transistors P11 and P21 and attain the fast stable operation, even if the basic 6 transistors have the similar current capability.
Also, according to the memory cell 101 of the first conventional example, since the resistance section R100 is connected in series to the load transistors P11 and P21, the voltages of the sources of the load transistors P11 and P21 are downed. As a result, the current capability of the load transistors P11 and P21 can be made smaller than those of the drive transistors N12 and N22 and access transistors N11 and N21.
Also, according to the memory cell 101 of the first conventional memory cell 101, the resistance section R100 is connected through the wiring L100 to the plurality of memory cells 101 connected to the digit lines DTi and DBi. Thus, the employment of one resistance section R100 can improve the operations of the plurality of memory cells 101.
However, in the memory cell 101 of the first conventional example, the resistance section R100 is connected through the wiring L100 to the plurality of memory cells 101 which are connected to the digit lines DTi and DBi. Therefore, the wiring L100 becomes long in order to connect the resistance section R100 and the plurality of memory cells 101 which are connected to the digit lines DTi and DBi. Thus, a large parasitic capacitance is generated. When a cell data as the voltage applied to the node QT of the memory cell 101 is rewritten from HIGH to LOW, the voltage of the node Q1 of the memory cell 101 must be reduced at the same time. However, due to the parasitic capacitance of the wiring L100 and the resistance section R100, the voltage of the node Q1 is reduced depending on an RC time constant. Thus, the fast writing operation is prevented.